Optical system and optical apparatus

ABSTRACT

The present invention is an optical apparatus, e.g. an image display apparatus, which enables observation of a clear image at a wide field angle with substantially no reduction in the brightness of the observation image, and which is extremely small in size and light in weight and hence unlikely to cause the observer to be fatigued. The optical apparatus has an image display device (6), and an ocular optical system (7) for leading an image of the image display device (6) to an observer&#39;s eyeball (1). The ocular optical system (7) includes, in order from the image side, a third surface (5) which forms an entrance surface, a first surface (3) which forms both a reflecting surface and an exit surface, and a second surface (4) which forms a reflecting surface. The first to third surfaces (3 to 5) are integrally formed with a medium put therebetween which has a refractive index larger than 1. A bundle of light rays emitted from the image display device (6) enters the ocular optical system (7) while being refracted by the third surface (5) and is internally reflected by the first surface (3) and reflected by the second surface (4). Then, the ray bundle is incident on the first surface (3) again and refracted thereby so as to be projected into the observer&#39;s eyeball with the observer&#39;s iris position or eyeball rolling center as the exit pupil (1).

This application is a divisional of my application Ser. No. 08/912,119,filed Aug. 15, 1997 now U.S. Pat. No. 5,875,056, which is a continuationof my application Ser. No. 08/505,516, filed Jul. 21, 1995, now U.S.Pat. No. 5,701,202, which issued Dec. 23, 1997 and which was based onJapanese Patent Application No. 120034/1995 filed May 18, 1995, thepriority of which is claimed for the present application, the content ofall three applications being incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an optical apparatus and, moreparticularly, to a head- or face-mounted image display apparatus thatcan be retained on the observer's head or face.

As an example of conventional head- or face-mounted image displayapparatus, an image display apparatus disclosed in Japanese PatentApplication Unexamined Publication (KOKAI) No. 3-101709 (1991) is known.FIG. 19(a) shows the entire optical system of the conventional imagedisplay apparatus, and FIG. 19(b) shows a part of an ocular opticalsystem used in the image display apparatus. As illustrated in thesefigures, in the conventional image display apparatus, an image that isdisplayed by an image display device is transmitted as an aerial imageby a relay optical system including a positive lens, and the aerialimage is projected into an observer's eyeball as an enlarged image by anocular optical system formed from a concave reflecting mirror.

U.S. Pat. No. 4,669,810 discloses another type of convention imagedisplay apparatus. In this apparatus, as shown in FIG. 20, an image of aCRT is transmitted through a relay optical system to form anintermediate image, and the image is projected into an observer's eye bya combination of a reflection holographic element and a combiner havinga hologram surface.

Japanese Patent Application Unexamined Publication (KOKAI) No. 62-214782(1987) discloses another type of conventional image display apparatus.As shown in FIGS. 21(a) and 21(b), the conventional image displayapparatus is designed to enable an image of an image display device tobe directly observed as an enlarged image through an ocular lens.

U.S. Pat. No. 4,026,641 discloses another type of conventional imagedisplay apparatus. In the conventional image display apparatus, as shownin FIG. 22, an image of an image display device is transferred to acurved object surface by an image transfer device, and the imagetransferred to the object surface is projected in the air by a toricreflector.

U.S. Reissued Pat. No. 27,356 discloses another type of conventionalimage display apparatus. As shown in FIG. 23, the apparatus is an ocularoptical system designed to project an object surface on an exit pupil bya semitransparent concave mirror and a semitransparent plane mirror.

However, an image display apparatus of the type in which an image of animage display device is relayed, as in the image display apparatusesshown in FIGS. 19(a), 19(b) and 20, must use several lenses as a relayoptical system in addition to an ocular optical system, regardless ofthe type of ocular optical system. Consequently, the optical path lengthincreases, and the optical system increases in both size and weight.

In a case where only the ocular optical system shown in FIG. 19(a) isused, since only a reflecting surface having a concave surface directedtoward the observer has positive power, as shown in FIG. 19(b), largenegative field curvature is produced as shown by reference symbol P1 inthe figure.

In a layout such as that shown in FIGS. 21(a) and 21(b), the amount towhich the apparatus projects from the observer's face undesirablyincreases. Further, since an image display device and an illuminationoptical system are attached to the projecting portion of the apparatus,the apparatus becomes increasingly large in size and heavy in weight.

Since a head-mounted image display apparatus is fitted to the humanbody, particularly the head, if the amount to which the apparatusprojects from the user's face is large, the distance from the supportingpoint on the head to the center of gravity of the apparatus is long.Consequently, the weight of the apparatus is imbalanced when theapparatus is fitted to the observer's head. Further, when the observermoves or turns with the apparatus fitted to his/her head, the apparatusmay collide with something.

That is, it is important for a head-mounted image display apparatus tobe small in size and light in weight. An essential factor in determiningthe size and weight of the apparatus is the layout of the opticalsystem.

However, if an ordinary magnifier alone is used as an ocular opticalsystem, exceedingly large aberrations are produced, and there is nodevice for correcting them. Even if spherical aberration can becorrected to a certain extent by forming the configuration of theconcave surface of the magnifier into an aspherical surface, otheraberrations such as coma and field curvature remain. Therefore, if thefield angle is increased, the image display apparatus becomesimpractical. Alternatively, if a concave mirror alone is used as anocular optical system, it is necessary to use not only ordinary opticalelements (lens and mirror) but also a device for correcting fieldcurvature by an image transfer device (fiber plate) having a surfacewhich is curved in conformity to the field curvature produced, as shownin FIG. 22.

In a coaxial ocular optical system in which an object surface isprojected on an observer's pupil by using a semitransparent concavemirror and a semitransparent plane mirror, as shown in FIG. 23, sincetwo semitransparent surfaces are used, the brightness of the image isreduced to as low a level as 1/16, even in the case of a theoreticalvalue. Further, since field curvature that is produced by thesemitransparent concave mirror is corrected by curving the objectsurface itself, it is difficult to use a flat display, e.g. an LCD(Liquid Crystal Display), as an image display device.

SUMMARY OF THE INVENTION

In view of the above-described problems of the conventional techniques,an object of the present invention is to provide an optical apparatus,e.g. an image display apparatus, which enables observation of a clearimage at a wide field angle with substantially no reduction in thebrightness of the observation image, and which is extremely small insize and light in weight and hence unlikely to cause the observer to befatigued.

To attain the above-described object, the present invention provides anoptical apparatus which has a device for forming an image to beobserved, and an ocular optical system for leading the image to anobserver's eyeball. The ocular optical system includes, in order fromthe image side, a third surface which forms an entrance surface, a firstsurface which forms both a reflecting surface and an exit surface, and asecond surface which forms a reflecting surface. The first to thirdsurfaces are integrally formed with a medium put therebetween which hasa refractive index larger than 1.

In this case, the optical apparatus may be used as a finder opticalsystem in which the image forming device is an objective lens systemhaving positive refractive power as a whole and designed to form anobject image, and in which the object image is formed in a space betweenthe objective lens system and the ocular optical system. Alternatively,the image forming device may be an image display device for forming animage for observation. In this case, the optical apparatus may have adevice for fitting both the image display device and the ocular opticalsystem to the observer's head.

In addition, the present invention provides an optical apparatus whichhas a device for displaying an image, and an ocular optical system forprojecting an image formed by the image display device and for leadingthe image to an observer's eyeball. The ocular optical system has atleast three surfaces, and a space that is formed by these surfaces isfilled with a medium having a refractive index larger than 1. The atleast three surfaces include, in order from the observer's eyeball sidetoward the image display device, a first surface serving as both arefracting surface and an internally reflecting surface, a secondsurface serving as a reflecting surface of positive power which facesthe first surface and is decentered or tilted with respect to anobserver's visual axis, and a third surface serving as a refractingsurface closest to the image display device. At least two of the atleast three surfaces have a finite curvature radius.

In addition, the present invention provides an optical apparatus whichhas a device for displaying an image, and an ocular optical system forprojecting an image formed by the image display device and for leadingthe image to an observer's eyeball. The ocular optical system includes adecentered optical element having at least three surfaces, and a spacewhich is formed by these surfaces is filled with a medium having arefractive index larger than 1. The at least three surfaces include,from the observer's eyeball side toward the image display device, afirst surface serving as both a refracting surface and a totallyreflecting surface, a second surface serving as a reflecting surface ofpositive power which faces the first surface and is decentered or tiltedwith respect to an observer's visual axis, and a third surface servingas a refracting surface closest to the image display device. At leasttwo of the at least three surfaces have a finite curvature radius. Theocular optical system further includes at least one optical surfacehaving refracting action. The decentered optical system and the at leastone optical surface are disposed in an optical path which extends fromthe image display device to the observer's eyeball.

The function of the above-described optical apparatuses of the presentinvention will be explained below. The following explanation will bemade on the basis of backward ray tracing in which light rays are tracedfrom the observer's pupil position toward the image display device forthe convenience of designing the optical system in a case where thepresent invention is used in an image display apparatus.

In the basic arrangement of the present invention, the optical apparatushas a device for forming an image to be observed, and an ocular opticalsystem for leading the image to an observer's eyeball. The ocularoptical system includes, in order from the image side, a third surfacewhich forms an entrance surface, a first surface which forms both areflecting surface and an exit surface, and a second surface which formsa reflecting surface. The first to third surfaces are integrally formedwith a medium put therebetween which has a refractive index largerthan 1. In backward ray tracing that is carried out from the observer'seyeball side, the sequence of the surface Nos. is as follows: firstsurface → second surface → first surface → third surface.

The reason for adopting the above-described arrangement will beexplained below. If the first to third surface are formed fromindependent optical elements, since it is demanded to dispose theseoptical elements with extremely high accuracy in terms of angle,distance, etc., assembly is difficult, and productivity degrades.Therefore, in the present invention, the first to third surfaces areintegrally formed (e.g. into a prism), thereby facilitating the assemblyoperation, and thus enabling an improvement in productivity.

Incidentally, if the optical system is used as a finder optical systemin which the image forming device for allowing image-forming rays toenter the ocular optical system comprises an objective lens systemhaving positive refractive power as a whole and designed to form anobject image, and in which the object image is formed in a space betweenthe objective lens system and the ocular optical system, the presentinvention can be used in a single-lens reflex camera, a compact camerain which an observation optical system and a finder optical system areprovided separately from each other, an electronic camera in which anelectronic imaging device is used in place of film, etc.

Of course, the optical apparatus can be applied to a head-mounted imagedisplay apparatus by using an image display device for forming an imagefor observation as the image forming device, and providing a device forfitting both the image display device and the ocular optical system tothe observer's head.

It should be noted that the ocular optical system can also be formed bycementing together a plurality of mediums having different refractiveindices which are larger than 1. This is an arrangement in which acemented surface is added inside a lens (prism) for the purpose ofcorrecting chromatic and other aberrations. By doing so, aberrationcorrection can be realized without interfering with facilitation ofassembly.

In the present invention, a space that is formed by the first, secondand third surfaces of the ocular optical system is filled with a mediumhaving a refractive index larger than 1, and two of the three surfacesare provided with a finite curvature radius, thereby making it possibleto correct spherical aberration, coma and field curvature produced bythe second surface, which is decentered or tilted, and thus succeedingin providing the observer a clear observation image having a wide exitpupil diameter and a wide field angle.

Concave mirrors generally have such nature that, if strong power isgiven to the concave surface, the Petzval sum increases, and positivefield curvature is produced. In addition, negative comatic aberration isproduced. By filling the space formed by the first, second and thirdsurfaces with a medium having a refractive index larger than 1, lightrays from the pupil are refracted by the first surface, and it istherefore possible to minimize the height at which extra-axial principaland subordinate rays are incident on the second surface. Thus, since theheight of the principal ray is low, the size of the second surface isminimized, and thus the ocular optical system can be formed in a compactstructure. Alternatively, the field angle can be widened. Further, sincethe height of the subordinate rays is reduced, it is possible tominimize comatic aberrations produced by the second surface,particularly higher-order comatic aberrations.

In a case where two of the three surfaces of the ocular optical systemhave a finite curvature radius, if the first surface has a finitecurvature radius in addition to the second surface, and it has positivepower, light rays are refracted by the first surface to a large extent.Therefore, the height of light rays incident on the second surface canbe further reduced. By this action, it is possible to reduce strongnegative comatic aberration produced by the second surface, which is aconcave mirror. In a case where the first surface has negative power, itcan effectively correct comatic aberration and field curvature which areproduced by the second surface when light rays are internally reflectedby the first surface after being reflected by the second surface.

In a case where the third surface has a finite curvature radius inaddition to the second surface, if the third surface is provided withnegative power, it becomes possible to correct field curvature producedby the second surface in particular.

Further, unlike a conventional arrangement in which an observation imageof an image display device is formed in the air as a real intermediateimage by a relay optical system and projected into an eyeball as anenlarged image by an ocular optical system, the optical apparatus of thepresent invention is arranged to project the image of the image displaydevice directly into an observer's eyeball as an enlarged image, therebyenabling the observer to see the enlarged image of the image displaydevice as a virtual image. Accordingly, the optical system can be formedfrom a relatively small number of optical elements. Further, since thesecond surface of the ocular optical system, which is a reflectingsurface, can be disposed immediately in front of the observer's face ina configuration conformable to the curve of his/her face, the amount towhich the optical system projects from the observer's face can bereduced to an extremely small value. Thus, a compact and light-weightimage display apparatus can be realized.

A very effective way of minimizing the size of the optical element andof improving the performance thereof is to arrange the system so thatinternal reflection of light rays at the first surface after beingreflected by the second surface is total reflection. This will beexplained below in detail.

FIGS. 14(a) and 14(b) are sectional views each illustrating an opticalray trace of the optical apparatus according to the present invention.FIG. 14(a) shows an ocular optical system in which a first surface 3does not totally reflect light rays. FIG. 14(b) shows an ocular opticalsystem in which total reflection occurs at a first surface 3. In thesesectional views, reference numeral 1 denotes an observer's pupilposition, 2 an observer's visual axis, 3 a first surface of an ocularoptical system, 4 a second surface of the ocular optical system, 5 athird surface of the ocular optical system, and 6 an image displaydevice. Reference numeral 7 denotes the ocular optical system having thefirst, second and third surfaces 3, 4 and 5. In FIG. 14(a), aninternally reflecting region M of the first surface 3 has beenmirror-coated. The other region of the first surface 3 is a refractingregion.

Light rays coming out of the pupil 1 are refracted by the first surface3 of the ocular optical system, reflected by the second surface 4, whichis a concave mirror, and internally reflected by the first surface 3.If, as shown in FIG. 14(a), there is a large difference between theheight at which upper extra-axial light rays U are reflected by thesecond surface 4 and the height at which the upper extra-axial lightrays U are reflected by the first surface 3 after being reflected by thesecond surface 4, the overall length of the ocular optical system 7correspondingly increases, resulting in an increase of the overall sizeof the ocular optical system 7. That is, as the difference between theheights of the reflection points decreases, the size of the ocularoptical system 7 can be made smaller. In other words, if the size of theocular optical system is kept constant, as the difference between theheights of the reflection points becomes smaller, the field angle can bewidened.

However, if the difference between the reflection heights of the upperextra-axial light rays U at the second surface 4 and the first surface 3is reduced in the ocular optical system of the present invention, asshown in FIG. 14(b), the upper light rays U are reflected at a positionhigher than a position at which lower extra-axial light rays L areincident on the first surface 3. Accordingly, when the first surface 3is not a totally reflecting surface, the refracting region of the firstsurface 3 overlaps the mirror coat region M'. Consequently, the lowerlight rays L are undesirably blocked.

That is, if the internal reflection at the first surface 3 satisfies thecondition for total reflection, the first surface 3 need not bemirror-coated. Therefore, even if the upper light rays U afterreflection at the second surface 4 and the lower light rays L incidenton the first surface 3 interfere with each other at the first surface 3,the upper and lower light rays U and L can perform their originalfunctions.

At the second surface 4, which is a decentered concave mirror, as thereflection angle becomes larger, comatic aberration occurs to a largerextent. However, in a case where light rays are totally reflected by thefirst surface 3, the angle of reflection at the second surface 4 can bereduced. Therefore, it is possible to effectively suppress theoccurrence of comatic aberration at the second surface 4.

It should be noted that, when the internal reflection at the firstsurface 3 does not satisfy the condition for total reflection, theinternally reflection region M of the first surface 3 needs to bemirror-coated.

Further, an effective way of correcting aberration is to form any one ofthe first, second and third surfaces of the ocular optical system into adecentered aspherical surface.

The above is an important condition for correcting comatic aberrations,particularly higher-order comatic aberrations and coma flare, producedby the second surface, which is decentered in a direction Y in acoordinate system (X, Y, Z), described later, or tilted with respect tothe visual axis.

In an image display apparatus which uses an ocular optical system of thetype having a decentered or tilted reflecting surface in front of anobserver's eyeball as in the present invention, light rays are obliquelyincident on the reflecting surface even on the observer's visual axis.Therefore, comatic aberration is produced. The comatic aberrationincreases as the inclination angle of the reflecting surface becomeslarger. However, if it is intended to realize a compact and wide-fieldimage display apparatus, it is difficult to ensure an observation imagehaving a wide field angle unless the amount of eccentricity(decentration) or the angle of inclination is increased to a certainextent because of the interference between the observer's head and theoptical path, or between the image display device and the optical path.Accordingly, as the field angle of an image display apparatus becomeswider and the size thereof becomes smaller, the inclination angle of thereflecting surface becomes larger. As a result, how to correct comaticaberration becomes a serious problem.

To correct such complicated comatic aberration, any one of the first,second and third surfaces constituting the ocular optical system isformed into a decentered aspherical surface. By doing so, the power ofthe optical system can be made asymmetric with respect to the visualaxis. Further, the effect of the aspherical surface can be utilized foroff-axis aberration. Accordingly, it becomes possible to effectivelycorrect comatic aberrations, including axial aberration.

Further, it is important that any one of the first, second and thirdsurfaces of the ocular optical system should be an anamorphic surface.That is, any one of the three surfaces should be a surface in which thecurvature radius in the YZ-plane and the curvature radius in theXZ-plane, which perpendicularly intersects the YZ-plane, are differentfrom each other.

The above is a condition for correcting aberration which occurs becausethe second surface is decentered or tilted with respect to the visualaxis. In general, if a spherical surface is decentered, the curvaturerelative to light rays incident on the surface in the plane of incidenceand that in a plane perpendicularly intersecting the incidence planediffer from each other. Therefore, in an ocular optical system where areflecting surface is disposed in front of an observer's eyeball in sucha manner as to be decentered or tilted with respect to the visual axisas in the present invention, an image on the visual axis lying in thecenter of the observation image is also astigmatically aberrated for thereason stated above. In order to correct the axial astigmatism, it isimportant that any one of the first, second and third surfaces of theocular optical system should be formed so that the curvature radius inthe plane of incidence and that in a plane perpendicularly intersectingthe incidence plane are different from each other.

Incidentally, in a case where a vertical plane containing an observer'svisual axis is defined as YZ-plane, and a horizontal plane containingthe observer's visual axis is defined as XZ-plane, it is preferable tosatisfy the following condition:

    0.5<R.sub.y2 /R.sub.x2 <5                                  (1)

where R_(y2) is the curvature radius of the second surface in theYZ-plane (i.e. the curvature radius of a surface intersecting a straightlight (visual axis) which is an extension of the principal ray extendingfrom the observer's pupil in the vicinity of the intersection), andR_(x2) is the curvature radius of the second surface in the XZ-plane(i.e. the curvature radius of a surface intersecting a straight line(visual axis) which is an extension of the principal ray extending fromthe observer's pupil in the vicinity of the intersection).

The above expression (1) is a condition for correcting aberrations,particularly axial and other astigmatic aberrations. In general, as thefield angle becomes larger, higher-order astigmatic aberrations appear.In a convex lens system, as the field angle becomes larger, themeridional image increases in the negative direction, whereas thesagittal image increases in the positive direction. In order to correctthese astigmatic aberrations, it is necessary to arrange the opticalsystem such that the power in the meridional plane (i.e. a cross-sectiontaken in a direction parallel to the plane of the figure) is reduced,whereas the power in the sagittal plane (i.e. a cross-section taken in adirection perpendicular to the plane of the figure) is increased.Accordingly, with regard to the curvature radius in one plane, thecurvature radius should be increased in the direction Y and reduced inthe direction X.

If the value of R_(y2) /R_(x2) falls outside the range of the condition(1), the degree of difference between the focus position of a ray bundlein a cross-section taken in a direction perpendicular to the plane ofthe figure and the focus position of a ray bundle in a cross-sectiontaken in a direction parallel to the plane of the figure becomesexcessively large beyond the observer's tolerance, making it exceedinglydifficult for the observer to see an image sent from the image displaydevice. That is, if the value of R_(y2) /R_(x2) is not smaller than theupper value, i.e. 5, the focal length at the focus position of the raybundle in the cross-section taken in a direction parallel to the planeof the figure becomes excessively longer than that at the focus positionof the ray bundle in the cross-section taken in a directionperpendicular to the plane of the figure. If the value of R_(y2) /R_(x2)is not larger than the lower limit, i.e. 0.5, the focal length at thefocus position of the ray bundle in the cross-section taken in adirection parallel to the plane of the figure becomes excessivelyshorter than that at the focus position of the ray bundle in thecross-section taken in a direction perpendicular to the plane of thefigure.

With regard to the lower limit of the condition (1), it is even moredesirable to set R_(y2) /R_(x2) to a value not less than 1 (i.e.1≦R_(y2) /R_(x2)). By doing so, it is possible for the observer to see aclearer image of the image display device. With regard to the upperlimit of the condition (1), it is even more desirable to set R_(y2)/R_(x2) to a value not larger than 2 (i.e. R_(y2) /R_(x2) ≦2). By doingso, it is possible for the observer to see a clearer image of the imagedisplay device.

In the ocular optical system of the present invention, a principalsurface having positive power is the second surface, which is areflecting surface. Therefore, it is preferably for the second surfaceto satisfy the condition (1) rather than for another surface to have adifference between the curvature radii in the YZ- and XZ-planes. Thatis, astigmatism correction can be made even more effective by allowingthe second surface to satisfy the condition (1). This is preferable interms of aberration correction.

Incidentally, an effective way of correcting aberration is to form thefirst surface into a transparent reflecting surface having a convexsurface directed toward the second surface. Since the second surface isa principal reflecting surface having positive power in the whole ocularoptical system, it produces field curvature to a considerable extent inaddition to the above-described comatic aberration. The negative comaticaberration produced by the second surface can be corrected by allowingthe first surface to have negative power so that the first surfaceproduces comatic aberration which is opposite in sign to the comaticaberration produced by the second surface. The positive field curvatureproduced by the second surface can be simultaneously corrected byproducing negative field curvature at the third surface.

It is also preferable to satisfy the following condition:

    -10<r.sub.1 /r.sub.2 <10                                   (2)

where r₁ is the curvature radius of the first surface of the ocularoptical system in the vicinity of an intersection between the observer'svisual axis and the first surface, and r₂ is the curvature radius of thesecond surface of the ocular optical system in the vicinity of anintersection between the observer's visual axis and the second surface.

It should be noted that r₁ and r₂ are curvature radii measured in thesame cross-section as in the case of the above R_(y2) (i.e. curvatureradii in the plane of the figure).

The above expression (2) is a condition for suppressing the occurrenceof field curvature by adjusting the curvature radii of the first andsecond surfaces on the observer's visual axis. If the value of r₁ /r₂ inthe condition (2) is not larger than the lower limit, i.e. -10, Petzvalsum increases toward the negative side, causing positive fieldcurvature. If the value of r₁ /r₂ in the condition (2) is not smallerthan the upper limit, i.e. 10, Petzval sum increases toward the positiveside, causing negative field curvature. It is even more desirable todefine the upper limit of the condition (2) as a value smaller than 5(i.e. r₁ /r₂ <5). By doing so, the aberration performance can be furtherimproved. Further, it is even more desirable to define the lower limitof the condition (2) as a value larger than -5 (i.e. -5<r₁ /r₂). Bydoing so, the aberration performance can be further improved.

In order to allow the first surface to perform total reflection asinternal reflection, it is necessary to satisfy the condition thatreflection angles of all light rays at the first surface are not smallerthan the critical angle Θ_(r) =sin⁻¹ (1/n) (where n is the refractiveindex of a medium constituting the optical system). In the case ofn=1.5, for example, Θ_(r) =41.81, and a reflection angle not smallerthan it is necessary. This will be explained below with reference toFIGS. 15(a) and 15(b).

FIGS. 15(a) and 15(b) show a part of the ocular optical system in whichlight rays are first reflected by the second surface 4 and theninternally reflected by the first surface 3. FIG. 15(a) shows the way inwhich reflection takes place when the first surface 3 is concave towardthe second surface 4. FIG. 15(b) shows the way in which reflection takesplace when the first surface 3 is convex toward the second surface 4.

After being reflected by the second surface 4, each light ray isdirected downward at a certain reflection angle. In a case where thefirst surface 3 is a reflecting surface having a concave surfacedirected toward the second surface 4, as shown in FIG. 15(a), lines Snormal to the first surface 3 convergently extend toward the secondsurface 4. Since a lower light ray L reflected by the second surface 4is incident on the first surface 3 in a direction along the line normalto the first surface 3, the reflection angle γ at the first surface 3cannot be made large. That is, it is difficult to satisfy the conditionfor total reflection with respect to all light rays reflected by thefirst surface 3. Conversely, in a case where the first surface 3 isconvex toward the second surface 4, as shown in FIG. 15(b), lines S'normal to the first surface 3 divergently extend toward the secondsurface 4. Accordingly, the reflection angle γ' can be effectivelyincreased even for the lower light ray. Thus, the condition for totalreflection at the first surface 3 can be readily satisfied at a widefield angle.

Further, it is desirable for either one of the first and third surfacesof the ocular optical system to be tilted or decentered with respect tothe visual axis. By tilting or decentering either one of the first andthird surfaces, it becomes possible to correct comatic aberrationsasymmetrically introduced into an image which lies closer to the imagedisplay device as viewed from the visual axis and into an image whichlies on the opposite side, and also possible to dispose the imagedisplay device on a plane which is approximately perpendicular to theoptical axis reflected by the second surface. This is effective when animage display device which is inferior in viewing angle characteristicsis used.

It is preferable to satisfy the following condition:

    10°≦α≦40°                (3)

where α is the angle between the visual axis 2 and the line normal tothe second surface 4 of the ocular optical system in the vicinity of anintersection between the visual axis 2 and the second surface 4.

This is a condition for disposing the ocular optical system and theimage display device 6 of the image display apparatus according to thepresent invention at appropriate positions. If the angle α is smallerthan the lower limit of the condition (3), i.e. 10°, the degree ofinclination of the second surface 4 becomes excessively small, so that abundle of light rays totally reflected by the first surface 3 isundesirably shifted downward of the visual axis 2 after being reflectedby the second surface 4. Consequently, light is incident on the lowerside of the observer's pupil 1, and thus it becomes impossible toobserve the image of the image display device 6 from the pupil 1. If theobserver forcefully shifts the pupil 1 downwardly in this state to seethe image of the image display device 6, aberrations in the plus andminus directions which are produced when light rays are reflected by thefirst and second surfaces 3 and 4 become incapable of canceling eachother with good balance. As a result, a distorted image is undesirablyobserved, and thus the performance is deteriorated. Conversely, if theangle α exceeds the upper limit of the condition (3), i.e. 40°, thedegree of inclination of the second surface 4 becomes excessively large,so that a bundle of light rays totally reflected by the first surface 3is undesirably shifted upward of the visual axis 2 after being reflectedby the second surface 4. Consequently, light is incident on the upperside of the observer's pupil 1, and thus it becomes impossible toobserve the image of the image display device 6 from the pupil 1. If theobserver forcefully shifts the pupil 1 upwardly in this state to see theimage of the image display device 6, aberrations in the plus and minusdirections which are produced when light rays are reflected by the firstand second surfaces 3 and 4 become incapable of canceling each otherwith good balance. As a result, a distorted image is undesirablyobserved, and thus the performance is deteriorated in the same way asthe above.

In order to achieve an even more effective layout, it is preferable todefine the lower limit for α as 20° or more. This is true of the upperlimit of the condition (3). That is, an even more effective layout isachieved by defining the upper limit for α as 30° or less.

Further, it is important that the display surface of the image displaydevice should be tilted with respect to the visual axis. In a case wherea refracting surface or a reflecting surface which constitutes anoptical element is decentered or tilted, the refraction or reflectionangle of light rays from the pupil at the refracting or reflectingsurface may vary according to the image height, causing the imagesurface to be tilted with respect to the visual axis. In such a case,the inclination of the image surface can be corrected by tilting thedisplay surface of the image display device with respect to the visualaxis.

Incidentally, as the field angle of an image display apparatus widensand the size thereof decreases, the inclination angle of the secondsurface, which is the first reflecting surface, increases, andhigher-order comatic aberrations produced thereby increase. Further,astigmatism that is produced by the inclination of the surface alsoincreases. Accordingly, it may be difficult to satisfactorily correctthese aberrations by only the decentered optical element in which aspace formed by at least three surfaces is filled with a medium having arefractive index larger than 1, and in which at least two of the atleast three surfaces have a finite curvature radius.

Therefore, at least one optical surface having refracting action isdisposed, in addition to the above-described decentered optical element,between the observer's eyeball and the image display device, therebymaking it possible to correct aberrations produced in the ocular opticalsystem even more effectively.

In the decentered optical element of the present invention, the secondsurface and the internally reflecting surface of the first surface,which is subsequent to the second surface, are reflecting surfaces.Therefore, no chromatic aberration is produced at these surfaces.Further, at the third surface, which lies in close proximity to theimage display device, the principal ray is approximately parallel to theoptical axis. Therefore, the third surface produces minimal chromaticaberration. Consequently, chromatic aberration produced by the firstsurface serving as a refracting surface is dominant in the ocularoptical system. Further, in a wide-field optical system such as that inthe present invention, lateral chromatic aberration appears moremarkedly than axial chromatic aberration. That is, it is important tocorrect lateral chromatic aberration produced by the first surface, andit is possible to display an image which is clearer and of higherresolution by correcting the lateral chromatic aberration. Accordingly,the ocular optical system is preferably arranged such that thedecentered optical element, together with at least one optical surfacehaving refracting action, is disposed between the observer's eyeball andthe image display device. By doing so, optical elements constituting theocular optical system can be composed of two or more different mediums,and it becomes possible to correct the lateral chromatic aberration byvirtue of the difference in Abbe's number between these mediums.

As has been described above, it is important in the ocular opticalsystem of the present invention to correct chromatic aberration producedby the first surface of the decentered optical element. The chromaticaberration can be corrected by forming the above-described at least oneoptical surface having refracting action from a surface which produceschromatic aberration which is approximately equal in quantity butopposite in sign to the chromatic aberration produced by the firstsurface.

The correction of chromatic aberration will be explained below morespecifically. By disposing the decentered optical element and at leastone optical surface having refracting action in the optical pathextending from the image display device to the observer's eyeball, theocular optical system can be composed of two or more different mediums.In this case, lateral chromatic aberration can be corrected by virtue ofthe Abbe's number difference between the different mediums. Forinstance, let us consider a case where the optical surface is disposedbetween the first and second surfaces of the decentered optical element,and the decentered optical element is composed of two different mediums.Achromatic conditions for the entire optical system are given by

    f.sub.1 =(ν.sub.1 -ν.sub.2)×f/ν.sub.1

    f.sub.2 =-(ν.sub.1 -ν.sub.2)×f/ν.sub.2

    1/f=1/f.sub.1 +1/f.sub.2

where f is the focal length of the entire optical system, f₁ is thefocal length of the first surface-side decentered optical element, ν₁ isthe Abbe's number of the first surface-side decentered optical element,f₂ is the focal length of the second surface-side decentered opticalelement, and ν₂ is the Abbe's number of the second surface-sidedecentered optical element.

The focal length f of the ocular optical system and the focal length f₂of the second surface-side decentered optical element are positive, andthe focal length f₁ of the first surface-side decentered optical elementis negative. Hence, the relationship between the Abbe's numbers of thefirst and second surface-side decentered optical elements is given by ν₁<ν₂. That is, by using a medium having a smaller Abbe's number to formthe first surface-side decentered optical element, chromatic aberrationcan be effectively corrected.

In a case where at least one optical surface is present at a positionother than the above, Abbe's numbers of the mediums can be set in thesame manner as in the above-described example.

In a case where the above-described at least one optical surface isdisposed between the observer's eyeball and the first surface of thedecentered optical element, and the optical surface has positiverefractive power, the beam diameter at the second surface of thedecentered optical element becomes small, and hence higher-order comaticaberrations reduce. Therefore, it is possible to observe a clear imageas far as the edges of the display surface of the image display device.Further, since a principal ray at the edge of the image is refracted bythe at least one optical surface having positive refractive power, theheight of the ray incident on the decentered optical system can bereduced. Therefore, it becomes possible to set a larger field angle thanin a case where the decentered optical system alone is used.

In a case where the above-described at least one optical surface isdisposed between the first and second surfaces of the decentered opticalelement, the decentered optical element is divided into first and secondsurface-side portions which are composed of two different mediums, ashas been described above; this is useful to correct chromaticaberration.

In a case where the above-described at least one optical surface isdisposed between the third surface of the decentered optical element andthe image display device, if the optical surface has negative power,since the position of the optical surface is closest to the imagedisplay device, it is possible to correct field curvature produced bythe decentered optical element.

By decentering the above-described at least one optical surface withrespect to the visual axis, it is possible to correct comaticaberrations asymmetrically introduced into an image which lies closer tothe image display device as viewed from the visual axis and into animage which lies on the opposite side, and also possible to allow theoptical axis to lie approximately perpendicular to a plane on which theimage display device is disposed.

By using a cemented lens to form the above-described at least oneoptical surface, lateral chromatic aberration produced in the decenteredoptical system can be corrected; this is useful to ensure a clearerimage and a wider field angle.

By forming the above-described at least one optical surface and thesurface of the decentered optical element that faces the optical surfaceinto concave surfaces, an air lens is formed. In this case, since thenegative powers of the two surfaces can be effectively utilized, Petzvalsum in the entire optical system can be minimized. Thus, field curvatureproduced by the second surface of the decentered optical element can beeffectively corrected.

It should be noted that it becomes possible for the observer to see astable observation image by providing a device for positioning both theimage display device and the ocular optical system with respect to theobserver's head.

By allowing both the image display device and the ocular optical systemto be fitted to the observer's head with a supporting device, it becomespossible for the observer to see the observation image in a desiredposture and from a desired direction.

Further, it becomes possible for the observer to see the observationimage with both eyes without fatigue by providing a device forsupporting at least two optical apparatuses at a predetermined spacing.Further, if images with a disparity therebetween are displayed on theright and left image display surfaces, and these images are observedwith both eyes, it is possible to enjoy viewing a stereoscopic image.

Further, if the optical apparatus is arranged to form an image of anobject at infinity with the image display device surface in the ocularoptical system defined as an image surface, the optical apparatus can beused as an imaging optical system, e.g. a finder optical system for acamera such as that shown in FIGS. 17 and 18.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical ray trace of Example 1 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 2 illustrates an optical ray trace of Example 2 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 3 illustrates an optical ray trace of Example 3 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 4 illustrates an optical ray trace of Example 4 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 5 illustrates an optical ray trace of Example 5 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 6 illustrates an optical ray trace of Example 6 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 7 illustrates an optical ray trace of Example 7 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 8 illustrates an optical ray trace of Example 8 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 9 illustrates an optical ray trace of Example 9 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 10 illustrates an optical ray trace of Example 10 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 11 illustrates an optical ray trace of Example 11 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 12 illustrates an optical ray trace of Example 12 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIG. 13 illustrates an optical ray trace of Example 13 in which theoptical apparatus of the present invention is applied to an imagedisplay apparatus.

FIGS. 14(a) and 14(b) are views used to explain internal reflection at afirst surface of the optical apparatus according to the presentinvention.

FIGS. 15(a) and 15(b) are views used to explain the relationship betweentotal reflection and the configuration of the first surface of theoptical apparatus according to the present invention.

FIGS. 16(a) and 16(b) are sectional and perspective views showing ahead-mounted image display apparatus according to the present invention.

FIG. 17 shows an arrangement in a case where the optical apparatus ofthe present invention is used as an imaging optical system.

FIG. 18 shows an arrangement in a case where the optical apparatus ofthe present invention is used as an imaging optical system.

FIGS. 19(a) and 19(b) show the optical system of a conventional imagedisplay apparatus.

FIG. 20 shows the optical system of another conventional image displayapparatus.

FIGS. 21(a) and 21(b) show the optical system of still anotherconventional image display apparatus.

FIG. 22 shows the optical system of a further conventional image displayapparatus.

FIG. 23 shows the optical system of a still further conventional imagedisplay apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples 1 to 13 in which the optical apparatus of the present inventionis applied to an image display apparatus will be described below withreference to FIGS. 1 to 13, which are sectional views of image displayapparatuses designed for a single eye according to Examples 1 to 13.

Constituent parameters of each example will be shown later. In thefollowing description, the parameter numbers are shown as ordinalnumbers in backward tracing from an observer's pupil position 1 towardan image display device 6. A coordinate system is defined as follows: Asshown in FIG. 1, with the observer's iris position 1 defined as theorigin, the direction of an observer's visual axis 2 is taken as Z-axis,where the direction toward an ocular optical system 7 from the origin isdefined as positive direction, and the vertical direction (as viewedfrom the observer's eyeball) which perpendicularly intersects theobserver's visual axis 2 is taken as Y-axis, where the upward directionis defined as position direction. Further, the horizontal direction (asviewed from the observer's eyeball) which perpendicularly intersects theobserver's visual axis 2 is taken as X-axis, where the leftwarddirection is defined as positive direction. That is, the plane of thefigure is defined as YZ-plane, and a plane which is perpendicular to theplane of the figure is defined as XZ-plane. The optical axis is bent inthe YZ-plane, which is parallel to the plane of the figure.

In the constituent parameters (shown later), regarding each surface forwhich eccentricities Y and Z and inclination angle Θ are shown, theeccentricity Y is a distance by which the vertex of the surfacedecenters in the Y-axis direction from the parameter number 1 (pupilposition 1), which is the reference surface, and the eccentricity Z is adistance by which the vertex of the surface decenters in the Z-axisdirection from the parameter number 1. The inclination angle Θ is theangle of inclination of the central axis of the surface from the Z-axis.In this case, positive Θ means counterclockwise rotation. It should benoted that a surface without indication of eccentricities Y, Z andinclination angle Θ is coaxial with respect to the preceding surface.

Regarding surface separations, the surface separation of the surface 3of parameter number 2 is the distance from the surface 3 of parameternumber 1 along the Z-axis direction, and a point on the surface 3 thatlies on the Z-axis is defined as a reference point. A point whichdecenters from the reference point in the direction Y by the giveneccentricity is the vertex of the surface 3 of parameter number 2.Regarding the coaxial portion, the surface separation is the axialdistance from the surface concerned to the next surface. It should benoted that surface separations are shown with the direction of backwardtracing along the optical axis defined as positive direction.

The non-rotationally symmetric aspherical configuration of each surfacemay be expressed in the coordinate system defining the surface asfollows: ##EQU1##

where R_(y) is the paraxial curvature radius of each surface in theYZ-plane (the plane of the figure); R_(x) is the paraxial curvatureradius in the XZ-plane; K_(x) is the conical coefficient in theXZ-plane; K_(y) is the conical coefficient in the YZ-plane; AR and BRare 4th- and 6th-order aspherical coefficients, respectively, which arerotationally symmetric with respect to the Z-axis; and AP and BP are4th- and 6th-order aspherical coefficients, respectively, which arerotationally asymmetric with respect to the Z-axis.

The rotationally symmetric aspherical configuration of each surface maybe expressed by

    Z=[(h.sup.2 /R)/[1+{1-(1+K)(h.sup.2 /R.sup.2)}.sup.1/2 ]+Ah.sup.4 +Bh.sup.6

where R is the paraxial curvature radius; K is the conical coefficient;A and B are 4th- and 6th-order aspherical coefficients, respectively;and h is h² =X² +Y².

It should be noted that the refractive index of the medium between apair of surfaces is expressed by the refractive index for the spectrald-line. Lengths are given in millimeters.

The following examples are all image display apparatuses for the righteye. An image display apparatus for the left eye can be realized bydisposing the constituent optical elements of each example insymmetrical relation to the apparatus for the right eye with respect tothe YZ-plane.

In an actual apparatus, needless to say, the direction in which theoptical axis is bent by the ocular optical system may be any of theupward, downward and sideward directions of the observer.

In each sectional view, reference numeral 1 denotes an observer's pupilposition, 2 an observer's visual axis, 3 a first surface of an ocularoptical system, 4 a second surface of the ocular optical system, 5 athird surface of the ocular optical system, and 6 an image displaydevice. Reference numeral 7 denotes the ocular optical system having thefirst, second and third surfaces 3, 4 and 5. Reference numeral 9 denotesan optical surface.

The actual path of light rays in each example is as follows: In Example1, for instance, a bundle of light rays emitted from the image displaydevice 6 enters the ocular optical system 7 while being refracted by thethird surface 5 of the ocular optical system 7 and is internallyreflected by the first surface 3 and reflected by the second surface 4.Then, the ray bundle is incident on the first surface 3 again andrefracted thereby so as to be projected into the observer's eyeball withthe observer's iris position or eyeball rolling center as the exit pupil1.

EXAMPLE 1

In this example, as shown in the sectional view of FIG. 1, thehorizontal field angle is 40°, while the vertical field angle is 30.6°,and the pupil diameter is 4 millimeters. In the constituent parameters(shown later), the parameter numbers 2, 3 and 4 correspond to anamorphicaspherical surfaces 3, 4 and 3, respectively, and the parameter number 5is a spherical surface 5.

EXAMPLE 2

In this example, as shown in the sectional view of FIG. 2, thehorizontal field angle is 45°, while the vertical field angle is 34.5°,and the pupil diameter is 4 millimeters. In the constituent parameters(shown later), the parameter numbers 2, 3 and 4 correspond to anamorphicaspherical surfaces 3, 4 and 3, respectively, and the parameter number 5is a flat surface 5.

EXAMPLE 3

In this example, as shown in the sectional view of FIG. 3, thehorizontal field angle is 45°, while the vertical field angle is 34.5°,and the pupil diameter is 4 millimeters. In the constituent parameters(shown later), the parameter numbers 2, 3, 4 and 5 correspond toanamorphic aspherical surfaces 3, 4, 3, 5, respectively.

EXAMPLE 4

In this example, as shown in the sectional view of FIG. 4, thehorizontal field angle is 30°, while the vertical field angle is 22.7°,and the pupil diameter is 4 millimeters. In the constituent parameters(shown later), the parameter number 3 is an anamorphic asphericalsurface 4, the parameter number 2 and 4 are flat surfaces 3, and theparameter number 5 is a spherical surface 5.

EXAMPLE 5

In this example, as shown in the sectional view of FIG. 5, thehorizontal field angle is 30°, while the vertical field angle is 22.7°,and the pupil diameter is 4 millimeters. In the constituent parameters(shown later), the numbers 3 and 5 correspond to anamorphic asphericalsurfaces 4 and 5, and the parameter numbers 2 and 4 correspond tospherical surfaces 4 and 5.

EXAMPLE 6

In this example, as shown in the sectional view of FIG. 6, thehorizontal field angle is 30°, while the vertical field angle is 22.7°,and the pupil diameter is 4 millimeters. In the constituent parameters(shown later), the parameter number 3 is for anamorphic asphericalsurface 4, and the parameter numbers 2, 4 and 5 correspond to sphericalsurfaces 3, 3, and 5, respectively.

EXAMPLE 7

In this example, as shown in the sectional view of FIG. 7, thehorizontal field angle is 45°, while the vertical field angle is 34.5°,and the pupil diameter is 4 millimeters. In the constituent parameters(shown later), the parameter numbers 2, 3, 5, 6 and 7 corresponds tospherical surfaces 3, 9, 9, 3, 9, the parameter number 4 is ananamorphic aspheric surface 4, and the parameter number 8 is arotationally symmetrical aspherical surface 5. The optical surface 9 isdefined by the parameter Nos. 3, 5 and 7, which are respectivelydisposed between the surfaces 3 and 4 (parameter numbers 2 and 4),between the surfaces 4 and 3 (parameter numbers 4 and 6) and between theparameter numbers 6 and 8 of the ocular optical system (decenteredoptical element) 7. The optical surface 9 is decentered with respect tothe visual axis.

EXAMPLE 8

In this example, as shown in the sectional view of FIG. 8, thehorizontal field angle is 40°, while the vertical field angle is 30.5°,and the pupil diameter is 8 millimeters. In the constituent parameters(shown later), the parameter numbers 2, 3, 4, 6 and 7 correspond tospherical surfaces, and the parameter number 5 is an anamorphicaspherical surface 4. The optical surfaces 9 and 10 are formed from apositive lens which is defined by the parameter numbers 2 and 3,respectively. The positive lens is disposed between the ocular opticalsystem (decentered optical element) 7 and the observer's eyeball in sucha manner as to be decentered with respect to the visual axis.

EXAMPLE 9

In this example, as shown in the sectional view of FIG. 9, thehorizontal field angle is 30°, while the vertical field angle is 22.7°,and the pupil diameter is 8 millimeters. In the constituent parameters(shown later), the parameter numbers 3, 5 and 7 correspond to sphericalsurfaces 9, 9 and 5, and the parameter numbers 2, 4 and 6 correspond toanamorphic aspherical surfaces 3, 4 and 3. The optical surface 9 isdefined by the parameter numbers 3 and 5, which are respectivelydisposed between the surfaces corresponding parameter numbers 2 and 4and between the surfaces of parameters numbers 4 and 6 of the ocularoptical system (decentered optical element) 7. The optical surface 9 isdecentered with respect to the visual axis.

EXAMPLE 10

In this example, as shown in the sectional view of FIG. 10, thehorizontal field angle is 45°, while the vertical field angle is 34.5°,and the pupil diameter is 8 millimeters. In the constituent parameters(shown later), the parameter numbers 5 and 6 correspond to sphericalsurfaces 5 and 9, and the parameter numbers 2, 3 and 4 are anamorphicaspherical surfaces 3, 4 and 3. The optical surface 9 is defined by theparameter number 6. The optical surface 9 is cemented to the ocularoptical system (decentered optical system) 7 so as to form a negativelens in cooperation with the parameter number 5 of the ocular opticalsystem 7. The optical surface 9 is decentered with respect to the visualaxis.

EXAMPLE 11

In this example, as shown in the sectional view of FIG. 11, thehorizontal field angle is 30°, while the vertical field angle is 22.7°,and the pupil diameter is 4 millimeters. In the constituent parameters(shown later), the parameter numbers 2, 3, 4, 6 and 7 correspond tospherical surfaces, and the parameter number 5 is an anamorphicaspherical surface 4. The optical surfaces 9 and 10 are formed from apositive lens which is defined by the parameter numbers 2 and 3,respectively. The positive lens is disposed between the ocular opticalsystem (decentered optical element) 7 and the observer's eyeball in sucha manner as to be decentered with respect to the visual axis.

EXAMPLE 12

In this example, as shown in the sectional view of FIG. 12, an opticalelement 8 providing see-through function (hereinafter referred to as"see-through optical element 8) is cemented to the outside world side ofthe optical system 7 in Example 1 described above. The optical system inthis example is arranged to have substantially no refractive power as awhole with respect to a bundle of light rays from the outside world.Thus, an outside world image can be clearly observed. The second surface4 to which the see-through optical element 8 is cemented in this exampleis formed from a semitransparent surface which reflects a ray bundlefrom the image display device 6, and which transmits light from theoutside world.

Further, by providing a liquid crystal shutter 11 at the outer side ofthe see-through optical element 8, as shown in FIG. 12, three differentobservation conditions can be realized: When an image of the imagedisplay device 6 is to be observed, it is viewed with the liquid crystalshutter 11 closed. When an outside world image alone is to be observed,the liquid crystal shutter 11 is opened, and the image display device 6is turned off. When the image display device 6 is turned on to displayan image with the liquid crystal shutter 11 open, an outside world imagecan be observed superimposed on an electronic image.

Constituent parameters in the above-described see-through condition willbe shown later. Since the optical path from an LCD as the image displaydevice 6 is the same as that in Example 1, description thereof isomitted.

In the constituent parameters (shown later), values at the refractingsurfaces with respect to a bundle of light rays from the outside worldin front of the observer's eyeball are shown. The values at therefracting surfaces are also shown in the sequence of the refractingsurface 3 to the refracting surface 4 to the refracting surface 15, inbackward ray tracing from the exit pupil 1 in the same way as in theother examples. It should be noted that the configuration of the surface15 of the see-through optical element 8 on which outside world light isincident is made the same as the configuration of the first surface 3 ofthe ocular optical system 7, and that the surface of the see-throughoptical element 8 from which the outside world light emanates is madethe same as the configuration of the second surface 4 of the ocularoptical system 7.

EXAMPLE 13

In this example, as shown in the sectional view of FIG. 13, an opticalelement 8 for providing see-through function is disposed at the outsideworld side of the optical system 7 in the above-described Example 2 witha very narrow air spacing provided therebetween. In this example also,the optical system is arranged to have substantially no refractive powerwith respect to a bundle of light rays from the outside world, in thesame way as in Example 12. Thus, an outside world image can be clearlyobserved. The second surface 4 is formed from a semitransparent surfacewhich reflects a ray bundle from the image display device 6, and whichtransmits light from the outside world. Further, a liquid crystalshutter 11 is disposed at the outer side of the see-through opticalelement 8.

By virtue of the above-described arrangement, three differentobservation conditions can be realized in the same way as in Example 12.

Constituent parameters in the above-described see-through condition willbe shown later. Since the optical path from an LCD as the image displaydevice 6 is the same as that in Example 2, description thereof isomitted.

In the constituent parameters (shown later), values at the refractingsurfaces with respect to a bundle of light rays from the outside worldin front of the observer's eyeball are shown. The values at therefracting surfaces are also shown in the sequence of the refractingsurface 3 to the refracting surface 4 to the refracting surface 14 tothe refracting surface 15, in backward ray tracing from the exit pupil 1in the same way as in the other examples. It should be noted that theconfiguration of the surface 15 of the see-through optical element 8 onwhich outside world light is incident is made the same as theconfiguration of the first surface 3 of the ocular optical system 7, andthat the surface 14 of the see-through optical element 8 from which theoutside world light emanates is made the same as the configuration ofthe second surface 4 of the ocular optical system 7.

Although in Examples 12 and 13 a prism having curved surfaces decenteredwith respect to the visual axis is provided as a see-through opticalelement at the outside world side of the ocular optical system of thepresent invention in order to allow excellent see-through observation bythe ocular optical system, it is also possible to use a Fresnel or otherlens, an optical element having a diffraction optical surface, etc.

Constituent parameters of the above-described Examples 1 to 13 will beshown below.

    __________________________________________________________________________                         Refractive                                                                             Abbe's No.                                      Parameter                                                                           Radius of Surface                                                                            index    (Inclination                                    No.   curvature separation                                                                         (Eccentricity)                                                                         angle)                                          __________________________________________________________________________    Example 1                                                                     1     ∞ (pupil)                                                                         26.360                                                        2     R.sub.y -108.187                                                                             1.4922   57.50                                                 R.sub.x -73.105                                                                              Y -24.028                                                                              θ -14.70°                                K.sub.y 0                                                                     K.sub.x 0                                                                     AR 5.54186 × 10.sup.-7                                                  BR 8.17563 × 10.sup.-11                                                 AP -0.0804376                                                                 BP -1.37947                                                             3     R.sub.y -69.871                                                                              1.4922   57.50                                                 R.sub.x -60.374                                                                              Y 19.109 θ 36.66°                                 K.sub.y -0.136826                                                                            Z 33.339                                                       K.sub.x -0.123306                                                             AR -7.23291 × 10.sup.-11                                                BR -4.52937 × 10.sup.-12                                                AP 29.0752                                                                    BP -2.08536                                                             4     R.sub.y -108.187                                                                             1.4922   57.50                                                 R.sub.x -73.105                                                                              Y -24.028                                                                              θ -14.70°                                K.sub.y 0      Z 26.360                                                       K.sub.x 0                                                                     AR 5.54186 × 10.sup.-7                                                  BR 8.17563 × 10.sup.-11                                                 AP -0.0804376                                                                 BP -1.37947                                                             5     77.772         Y -35.215                                                                              θ -47.77°                                               Z 18.817                                                 6     (display device)                                                                             (from No. 1 surface)                                                          Y -30.892                                                                              θ -52.77°                                               Z 43.084                                                 (1)   R.sub.y2 /R.sub.x2 = 1.157                                              (2)   r.sub.1 /,r.sub.2 = 1.55                                                (3)   α = 53.34°                                                 Example 2                                                                     1     ∞ (pupil)                                                                         20.267                                                        2     R.sub.y -420.378                                                                             1.4922   57.50                                                 R.sub.x -99.789                                                                              Y -49.262                                                                              θ -13.94°                                K.sub.y 5.709616                                                              K.sub.x -2.785007                                                             AR 5.37533 × 10.sup.-7                                                  BR -6.41106 × 10.sup.-11                                                AP -0.422753                                                                  BP -0.455912                                                            3     R.sub.y -122.291                                                                             1.4922   57.50                                                 R.sub.x -69.335                                                                              Y -34.556                                                                              θ 37.99°                                 K.sub.y 0.774787                                                                             Z 24.367                                                       K.sub.x -0.104426                                                             AR -1.82945 × 10.sup.-9                                                 BR 4.45272 × 10.sup.-14                                                 AP 5.40431                                                                    BP -1.13468                                                             4     R.sub.y -420.378                                                                             1.4922   57.50                                                 R.sub.x -99.789                                                                              Y -49.262                                                                              θ -13.94°                                K.sub.y 5.709616                                                                             Z 20.267                                                       K.sub.x -2.785007                                                             AR 5.37533 × 10.sup.-7                                                  BR -6.41106 × 10.sup.-11                                                AP -0.422753                                                                  BP -0.455912                                                            5     ∞        Y -33.816                                                                              θ -56.84°                                               Z 21.726                                                 6     (display device)                                                                             Y -31.165                                                                              θ -50.91°                                               Z 38.433                                                 (1)   R.sub.y2 /R.sub.x2 = 1.764                                              (2)   r.sub.1 /r.sub.2 = 3.44                                                 (3)   α = 52.01°                                                 Example 3                                                                     1     ∞ (pupil)                                                                         19.657                                                        2     R.sub.y -178.469                                                                             1.4922   57.50                                                 R.sub.x -75.710                                                                              Y -42.983                                                                              θ -19.56°                                K.sub.y -4.700072                                                             K.sub.x -1.222689                                                             AR 9.71232 × 10.sup.-7                                                  BR -1.79187 × 10.sup.-10                                                AP -0.426826                                                                  BP -0.380615                                                            3     R.sub.y -81.632                                                                              1.4922   57.50                                                 R.sub.x -66.826                                                                              Y 30.011 θ 40.46°                                 K.sub.y -0.070545                                                                            Z 26.362                                                       K.sub.x -0.574123                                                             AR 3.90381 × 10.sup.-11                                                 BR -2.95604 × 10.sup.-14                                                AP -62.1044                                                                   BP 3.68602                                                              4     R.sub.y -178.469                                                                             1.4922   57.50                                                 R.sub.x -75.710                                                                              Y -42.983                                                                              θ -19.56°                                K.sub.y -4.700072                                                                            Z 19.657                                                       K.sub.x -1.222689                                                             AR 9.71232 × 10.sup.-7                                                  BR -1.79187 × 10.sup.-10                                                AP -0.426826                                                                  BP -0.380615                                                            5     R.sub.y -78.809                                                                              Y -28.629                                                                              θ -69.21°                                R.sub.x -15.380                                                                              Z 27.051                                                       K.sub.y -12.000                                                               K.sub.x -7.201382                                                             AR -9.38885 × 10.sup.-7                                                 BR -3.46619 × 10.sup.-9                                                 AP -0.995315                                                                  BP 0.706461                                                                   (display device)                                                                             Y -30.077                                                                              θ -55.73°                                               Z 38.578                                                 (1)   R.sub.y2 /R.sub.x2 = 1.222                                              (2)   r.sub.1 /r.sub.2 = 2.19                                                 (3)   α = 49.54°                                                 Example 4                                                                     1     ∞ (pupil)                                                                         35.674                                                        2     ∞        1.4870   70.40                                                                Y 4.942  θ 15.45°                           3     R.sub.y -166.785                                                                             1.4870   70.40                                                 R.sub.x -129.798                                                                             Y -2.361 θ 30.67°                                 K.sub.y 0.644353                                                                             Z 64.642                                                       K.sub.x -3.574565                                                             AR -1.34076 × 10.sup.-7                                                 BR -6.16761 × 10.sup.-13                                                AP -0.140999                                                                  BP -6.05079                                                             4     ∞        1.4870   70.40                                                                Y 4.942  θ 15.45°                                                Z 35.674                                                 5     92.827         Y -21.834                                                                              θ -50.80°                                               Z 78.827                                                 6     (display device)                                                                             Y -44.030                                                                              θ -8.14°                                                Z 74.025                                                 (1)   R.sub.y2 /R.sub.x2 = 1.285                                              (2)   r.sub.1 /r.sub.2 = -0.60                                                (3)   α = 59.33°                                                 Example 5                                                                     1     ∞ (pupil)                                                                         32.614                                                        2     359.756        1.4870   70.40                                                                Y 2.346  θ 9.84°                            3     R.sub.y -173.440                                                                             1.4870   70.40                                                 R.sub.x -140.501                                                                             Y -1.857 θ 29.87°                                 K.sub.y -8.751468                                                                            Z 58.946                                                       K.sub.x 4.994003                                                              AR 2.50178 × 10.sup.-8                                                  BR -1.79281 × 10.sup.-14                                                AP -3.8616                                                                    BP 23.172                                                               4     359.756        1.4870   70.40                                                                Y 2.346  θ 9.84°                                                 Z 32.614                                                 5     R.sub.y -71.035                                                                              Y -28.993                                                                              θ -50.31°                                R.sub.x -30.258                                                                              Z 64.366                                                       K.sub.y 0                                                                     K.sub.x -4.016232                                                             AR 1.64494 × 10.sup.-5                                                  BR -6.89738 × 10.sup.-9                                                 AP 0.757293                                                                   BP -0.0405894                                                           6     (display device)                                                                             Y -42.040                                                                              θ -9.06°                                                Z 63.493                                                 (1)   R.sub.y2 /R.sub.x2 = 1.234                                              (2)   r.sub.1 /r.sub.2 = -2.07                                                (3)   α = 60.13°                                                 Example 6                                                                     1     ∞ (pupil)                                                                         31.861                                                        2     624.447        1.4870   70.40°                                                        Y 4.081  θ 12.08°                           3     R.sub.y -205.155                                                                             1.4870   70.40                                                 R.sub.x -147.117                                                                             Y -0.579 θ 31.42°                                 K.sub.y 5.070131                                                                             Z 59.565                                                       K.sub.x -2.741334                                                             AR -9.17885 × 10.sup.-9                                                 BR -4.90794 × 10.sup.-13                                                AP 0.353607                                                                   BP -8.4008                                                              4     624.447        1.4870   70.40                                                                Y 4.081  θ 12.08°                                                Z 31.861                                                 5     62. 779        Y -30.891                                                                              θ -54.73°                                               Z 69.696                                                 6     (display device)                                                                             Y -46.009                                                                              θ -6.40°                                                Z 71.400                                                 (1)   R.sub.y2 /R.sub.x2 = 1.395                                              (2)   r.sub.1 /r.sub.2 = -3.04                                                (3)   α = 58.58°                                                 Example 7                                                                     1     ∞ (pupil)                                                                         25.798                                                        2     -96.979        1.7550   27.60                                                                Y 35.503 θ 19.41°                           3     -371.916       1.7184   46.86                                                                Y 28.466 θ 1.64°                                                 Z 34.179                                                 4     R.sub.y -73.443                                                                              1.7184   46.86                                                 R.sub.x -69.804                                                                              Y -27.105                                                                              θ -5.80°                                 K.sub.y 0.36532                                                                              Z 50.843                                                       K.sub.x -0.017813                                                             AR 2.37314 × 10.sup.-10                                                 BR 3.61091 × 10.sup.-12                                                 AP -8.04115                                                                   BP 0.142633                                                             5     -371.916       1.7550   27.60                                                                Y 28.466 θ 1.64°                                                 Z 34.179                                                 6     -96.979        1.7550   27.60                                                                Y 35.503 θ 19.41°                                                Z 25.789                                                 7     -371. 916      1.7184   46.86                                                                Y 28.466 θ 1.64°                                                 Z 34.171                                                 8     R -64.000      Y -5.065 θ -14.73°                                K 0.032998     Z 55.928                                                       A -2.03599 × 10.sup.-6                                                  B 8.44986 × 10.sup.-10                                            9     (display device)                                                                             Y -32.487                                                                              θ -42.92°                                               Z 46.354                                                 (1)   R.sub.y2 /R.sub.x2 = 1.052                                              (3)   α = 95.80°                                                 Example 8                                                                     1     ∞ (pupil)                                                                         5.434                                                         2     -67.198   12.671                                                                             1.4870   70.40                                                                Y -28.840                                                                              θ -62.14°                          3     -55.775                                                                 4     -108.280       1.7095   47.70                                                                Y 23.401 θ 20.84°                                                Z 45.112                                                 5     R.sub.y -81.008                                                                              1.7095   47.70                                                 R.sub.x -76.504                                                                              Y -20.800                                                                              θ 9.41°                                  K.sub.y 0.596647                                                                             Z 73.513                                                       K.sub.x 0.2904                                                                AR 6.598 × 10.sup.-8                                                    BR 7.20621 × 10.sup.-12                                                 AP -0.0350833                                                                 BP -0.148558                                                            6     -108.280       1.7095   47.70                                                                Y 23.401 θ 20.84°                                                Z 45.112                                                 7     -168.220       Y -4.594 θ -33.610                                                      Z 96.768                                                 8     (display device)                                                                             Y -44.345                                                                              θ -40.19°                                               Z 69.067                                                 (1)   R.sub.y2 /R.sub.x2 = 1.059                                              (2)   r.sub.1 /r.sub.2 = 1.34                                                 (3)   α = 80.59°                                                 Example 9                                                                     1     ∞ (pupil)                                                                         44.607                                                        2     R.sub.y -735.371                                                                             1.6792   51.00                                                 R.sub.x ∞                                                                              Y -18.782                                                                              θ -7.75°                                 K.sub.y 0                                                                     K.sub.x 0                                                                     AR -1.57554 × 10.sup.-8                                                 BR 9.32392 × 10.sup.-14                                                 AP -1.67996                                                                   BP 0.122856                                                             3     -61.610        1.6682   32.23                                                                Y 18.019 θ 29.91°                                                Z 60.673                                                 4     R.sub.y -151.581                                                                             1.6682   32.23                                                 R.sub.x -170.090                                                                             Y 23.479 θ 23.76°                                 K.sub.y -8.127909                                                                            Z 62.324                                                       K.sub.x 1.633055                                                              AR -5.16785 × 10.sup.-8                                                 BR 2.0965 × 10.sup.-12                                                  AP 0.595412                                                                   BP 0.638703                                                             5     -61.610        1.6792   51.00                                                                Y 18.019 θ 29.91°                                                Z 60.673                                                 6     R.sub.y -735.371                                                                             1.6792   51.00                                                 R.sub.x ∞                                                                              Y -18.782                                                                              θ -7.75°                                 K.sub.y 0      Z 44. 607                                                      K.sub.x 0                                                                     AR -1.57554 × 10.sup.-8                                                 BR 9.32392 × 10.sup.-14                                                 AP -1.67996                                                                   BP 0.122856                                                             7     -290.903       Y -40.650                                                                              θ -69.40°                                               Z 38.685                                                 8     (display device)                                                                             Y -44.086                                                                              θ -35.00°                                               Z 77.749                                                 (1)   R.sub.y2 /R.sub.x2 = 0.891                                              (3)   α = 66.24°                                                 Example 10                                                                    1     ∞ (pupil)                                                                         27.648                                                        2     R.sub.y -127.773                                                                             1.7394   45.06                                                 R.sub.x -74.145                                                                              Y -39.982                                                                              θ -19.01°                                K.sub.y 0                                                                     K.sub.x 0                                                                     AR 3.66126 × 10.sup.-7                                                  BR 1.819 × 10.sup.-11                                                   AP -0.356073                                                                  BP -0.699016                                                            3     R.sub.y -81.881                                                                              1.7394   45.06                                                 R.sub.x -68.040                                                                              Y 28.434 θ 39.91°                                 K.sub.y -0.111977                                                                            Z 37.099                                                       K.sub.x -0.354898                                                             AR 5.28817 × 10.sup.-12                                                 BR -3.7857 × 10.sup.-12                                                 AP 37.4793                                                                    BP -0.751459                                                            4     R.sub.y -127.773                                                                             1.7394   45.06                                                 R.sub.x -74.145                                                                              Y -39.982                                                                              θ -19.01°                                K.sub.y 0      Z 27.648                                                       K.sub.x 0                                                                     AR 3.66126 × 10.sup.-7                                                  BR 1.819 × 10.sup.-11                                                   AP -0.356073                                                                  BP -0.699016                                                            5     -21.067   1.567                                                                              1.7550   27.60                                                                Y -33.151                                                                              θ -59.68°                                               Z 46.009                                                 6     -66.347                                                                 7     (display device)                                                                             Y -36.167                                                                              θ -50.37°                                               Z 52.703                                                 (1)   R.sub.y2 /R.sub.x2 = 1.203                                              (2)   r.sub.1 /r.sub.2 = 1.56                                                 (3)   α = 50.09°                                                 Example 11                                                                    1     ∞ (pupil)                                                                         25.000                                                        2     123.041   6.500                                                                              1.5940   61.72                                                                Y -24.800                                                                              θ 5.787°                           3     -594.632                                                                4     239.449        1.4870   70.40                                                                Y -6.355 θ 14.22°                                                Z 34.191                                                 5     R.sub.y -408.357                                                                             1.4870   70.40                                                 R.sub.x -239.896                                                                             Y -11.547                                                                              θ 27.47°                                 K.sub.y -59.547081                                                                           Z 63.131                                                       K.sub.x 29.562822                                                             AR 2.289885 × 10.sup.-7                                                 BR 8.51773 × 10.sup.-11                                                 AP -0.538645                                                                  BP -0.20468                                                             6     239.449        1.4870   70.40                                                                Y -6.355 θ 34.19°                                                Z 34.191                                                 7     35.931         Y -36.256                                                                              θ -31.36°                                               Z 52.780                                                 8     (display device)                                                                             Y -36.901                                                                              θ -6.46°                                                Z 70.463                                                 (1)   R.sub.y2 /R.sub.x2 = 1.702                                              (3)   α = 62.53°                                                 Example 12                                                                    1 (1) ∞ (pupil)                                                                         26.360                                                        2 (3) R.sub.y -108.187                                                                             1.4922   57.50                                                 R.sub.x -73.105                                                                              Y -24.028                                                                              θ -14.70°                                K.sub.y 0                                                                     K.sub.x 0                                                                     AR 5.54186 × 10.sup.-7                                                  BR 8.17563 × 10.sup.-11                                                 AP -0.0804376                                                                 BP -1.37947                                                             3 (4) R.sub.y -69.871                                                                              1.4922   57.50                                                 R.sub.x -60.374                                                                              Y 19.109 θ 36.66°                                 K.sub.y -0.136826                                                                            Z 33.339                                                       K.sub.x -0.123306                                                             AR -7.23291 × 10.sup.-11                                                BR -4.52937 × 10.sup.-12                                                AP 29.0752                                                                    BP -2.08536                                                             4 (15)                                                                              R.sub.y -108.187                                                                             1.4922   57.50                                                 R.sub.x -73.105                                                                              Y -24.028                                                                              θ -14.70°                                K.sub.y 0      Z 48.339                                                       K.sub.x 0                                                                     AR 5.54186 × 10.sup.-7                                                  BR 8.17563 × 10.sup.-11                                                 AP -0.0804376                                                                 BP -1.37947                                                             Example 13                                                                    1 (1) ∞ (pupil)                                                                         20.267                                                        2 (3) R.sub.y -420.378                                                                             1.4922   57.50                                                 R.sub.x -99.789                                                                              Y -49.262                                                                              θ -13.94°                                K.sub.y 5.709616                                                              K.sub.x -2.785007                                                             AR 5.37533 × 10.sup.-7                                                  BR -6.41106 × 10.sup.-11                                                AP -0.422753                                                                  BP -0.455912                                                            3 (4) R.sub.y -122.291                                                                             Y -34.556                                                                              θ 37.99°                                 R.sub.x -69.335                                                                              Z 24.367                                                       K.sub.y 0.774787                                                              K.sub.x -0.104426                                                             AR -1.82945 × 10.sup.-9                                                 BR 4.45272 × 10.sup.-14                                                 AP 5.40431                                                                    BP -1.13468                                                             4 (14)                                                                              R.sub.y -122.291                                                                             1.4922   57.50                                                 R.sub.x -69.335                                                                              Y -34.556                                                                              θ 37.99°                                 K.sub.y 0.774787                                                                             Z 26.000                                                       K.sub.x -0.104426                                                             AR -1.82945 × 10.sup.-9                                                 BR 4.45272 × 10.sup.-14                                                 AP 5.40431                                                                    BP -1.13468                                                             5 (15)                                                                              R.sub.y -420.378                                                                             1.4922   57.50                                                 R.sub.x -99.789                                                                              Y -49.262                                                                              θ -13.94°                                K.sub.y 5.709616                                                                             Z 46.000                                                       K.sub.x -2.785007                                                             AR 5.37533 × 10.sup.-7                                                  BR -6.41106 × 10.sup.-11                                                AP -0.422753                                                                  BP -0.455912                                                            __________________________________________________________________________

Although Examples in which the optical apparatus of the presentinvention is applied to an image display apparatus have been describedabove, it should be noted that the present invention is not necessarilylimited to these Examples, and that various modifications may beimparted thereto. To arrange the optical apparatus of the presentinvention as a head-mounted image display apparatus (HMD) 13, as shownin the sectional view of FIG. 16(a) and the perspective view of FIG.16(b), the HMD 13 is fitted to the observer's head by using a headband10, for example, which is attached to the HMD 13. In this example ofuse, the HMD 13 may be arranged such that the second surface 2 of theocular optical system is formed by using a semitransparent mirror(half-mirror) 12, and a liquid crystal shutter 11 is provided in frontof the half-mirror 12, thereby enabling an outside world image to beselectively observed or superimposed on the image of the image displaydevice.

Further, the ocular optical system of the image display apparatusaccording to the present invention can be used as an imaging opticalsystem. For example, as shown in the perspective view of FIG. 17, theocular optical system may be used in a finder optical system F_(i) of acompact camera C_(a) in which a photographic optical system O_(b) andthe finder optical system F_(i) are provided separately in parallel toeach other. FIG. 18 shows the arrangement of an optical system in a casewhere the ocular optical system of the present invention is used as suchan imaging optical system. As illustrated, the ocular optical system DSof the present invention is disposed behind a front lens group GF and anaperture diaphragm D, thereby constituting an objective optical systemL_(t). An image that is formed by the objective optical system L_(t) iserected by a Porro prism P, in which there are four reflections,provided at the observer side of the objective optical system L_(t),thereby enabling an erect image to be observed through an ocular lensO_(c).

As will be clear from the foregoing description, the optical apparatusof the present invention makes it possible to provide an image displayapparatus which has a wide field angle and is extremely small in sizeand light in weight, and a novel imaging optical system.

What I claim is:
 1. An image-forming optical system which forms an imageof an object,said image-forming optical system comprising at least oneprism member, wherein said prism member has a first surface and a secondsurface, which face each other across a medium having a refractive index(n) larger than 1 (n>1), so that light rays entering said prism memberfrom an object side thereof are reflected at least twice in said prismmember, wherein both said first surface and said second surface arecurved surface configurations, and at least one of the curved surfaceconfigurations is formed from a rotationally asymmetric curved surface.2. An image-forming optical system according to claim 1, wherein saidprism member further has a third surface which faces said first surfaceand second surface across said medium.
 3. An image-forming opticalsystem according to claim 2, wherein said prism member is arranged suchthat both said first surface and said second surface have a reflectingaction, and said third surface has a transmitting action.
 4. Animage-forming optical system according to claim 3, wherein said firstsurface of said prism member has both a transmitting action and areflecting action.
 5. An image-forming optical system according to claim4, wherein said first surface of said prism member is formed from atotally reflecting surface so as to have said transmitting action andreflecting action.
 6. An image-forming optical system according to claim4, wherein said prism member is arranged such that light from the objectenters said prism member by passing through said first surface and isreflected by said second surface and further reflected by said firstsurface, and the reflected light exits from said prism member by passingthrough said third surface.
 7. An image-forming optical system accordingto any one of claims 1 to 6, wherein said rotationally asymmetric curvedsurface is formed from a configuration having an aberration correctingaction to correct decentration aberrations caused by reflection in saidprism member.
 8. An image-forming optical system according to claim 7,wherein said second surface is formed from said rotationally asymmetriccurved surface.
 9. An image-forming optical system according to claim 7,wherein said first surface is formed from said rotationally asymmetriccurved surface.
 10. An image-forming optical system according to claim7, wherein said third surface is formed from said rotationallyasymmetric curved surface.
 11. An image-forming optical system accordingto claim 7, wherein said prism member is provided with a mirror coatingso as to have a reflecting action.
 12. An image-forming optical systemaccording to claim 7, wherein said first surface is formed from aconfiguration having a concave surface directed toward the object. 13.An image-forming optical system according to claim 7, wherein saidsecond surface is formed from a configuration having a concave surfacedirected toward said medium.
 14. An image-forming optical systemaccording to claim 7, further comprising a lens disposed closer to theobject than said prism member.
 15. An image-forming optical systemaccording to claim 7, further comprising an aperture stop, wherein saidprism member is disposed between said aperture stop and said objectimage.
 16. An image-forming optical system according to claim 7, whereina field angle in a horizontal direction of said prism member isdifferent from a field angle in a vertical direction thereof.
 17. Animage-forming optical system according to claim 16, wherein the fieldangle in the horizontal direction of said prism member is larger thanthe field angle in the vertical direction thereof.
 18. A cameraapparatus according to claim 7, wherein said image-forming opticalsystem is disposed to perform image formation.
 19. A camera apparatusaccording to claim 18, wherein a photographic optical system and afinder optical system are disposed separately from each other.
 20. Acamera apparatus according to claim 19, wherein said image-formingoptical system is disposed in said finder optical system.
 21. A cameraapparatus according to claim 20, wherein said finder optical systemincludes, in order from an object side thereof, said image-formingoptical system; an image erecting optical system for erecting the objectimage formed by said image-forming optical system; and an ocular opticalsystem for observing said object image.
 22. An image-forming opticalsystem according to any one of claims 3 to 5, wherein said first surfaceis formed from a rotationally asymmetric curved surface having anaberration correcting action to correct decentration aberrations causedby reflection in said prism member, so that light from the object issubjected to said aberration correcting action when passing through saidfirst surface and is also subjected to said aberration correcting actionwhen reflected by said first surface, whereby double effect is producedby one surface.
 23. An ocular optical system arranged to lead an imageformed on an image plane to an observer's eyeball,said ocular opticalsystem comprising at least one prism member, wherein said prism memberhas a first surface and a second surface, which face each other across amedium having a refractive index (n) larger than 1 (n>1), so that lightrays entering said prism member from an image side thereof are reflectedat least twice in said prism member, wherein both said first surface andsaid second surface are curved surface configurations, and at least oneof the curved surface configurations is formed from a rotationallyasymmetric curved surface.
 24. An ocular optical system according toclaim 23, wherein said prism member further has a third surface whichfaces said first surface and second surface across said medium.
 25. Anocular optical system according to claim 24, wherein said prism memberis arranged such that both said first surface and said second surfacehave a reflecting action, and said third surface has a transmittingaction.
 26. An ocular optical system according to claim 25, wherein saidfirst surface of said prism member has both a transmitting action and areflecting action.
 27. An ocular optical system according to claim 26,wherein said prism member is arranged such that light from said imageenters said prism member by passing through said third surface and isreflected by said first surface and further reflected by said secondsurface, and the reflected light exits from said prism member by passingthrough said first surface.
 28. An ocular optical system according toclaim 26, wherein said first surface of said prism member is formed froma totally reflecting surface so as to have both said transmitting actionand reflecting action.
 29. An ocular optical system according to any oneof claims 26 to 28, wherein said first surface is formed from arotationally asymmetric curved surface having an aberration correctingaction to correct decentration aberrations caused by reflection in saidprism member, so that light from said image is subjected to saidaberration correcting action when passing through said first surface andis also subjected to said aberration correcting action when reflected bysaid first surface, whereby double effect is produced by one surface.30. An ocular optical system according to any one of claims 24 to 26,wherein said rotationally asymmetric curved surface is formed from aconfiguration having an aberration correcting action to correctdecentration aberrations caused by reflection in said prism member. 31.An ocular optical system according to claim 30, wherein said secondsurface is formed from said rotationally asymmetric curved surface. 32.An ocular optical system according to claim 30, wherein said firstsurface is formed from said rotationally asymmetric curved surface. 33.An ocular optical system according to claim 30, wherein said thirdsurface is formed from said rotationally asymmetric curved surface. 34.An ocular optical system according to claim 30, wherein said firstsurface is formed from a configuration having a concave surface directedtoward the observer's eyeball.
 35. An ocular optical system according toclaim 30, wherein said second surface is formed from a configurationhaving a concave surface directed toward said medium.
 36. An ocularoptical system according to claim 30, wherein a field angle in ahorizontal direction of said prism member is different from a fieldangle in a vertical direction thereof.
 37. An ocular optical systemaccording to claim 36, wherein the field angle in the horizontaldirection of said prism member is larger than the field angle in thevertical direction thereof.
 38. An ocular optical system according toclaim 24, wherein said prism member is provided with a mirror coating soas to have a reflecting action.
 39. In a finder optical systemcomprising an objective optical system for forming an object image; animage erecting optical system for erecting said object image; and anocular optical system for observing said object image;the improvementwhich comprises at least one prism member, wherein said prism member hastwo surfaces having a reflecting action to reflect the image in saidprism member, and two surfaces having a transmitting action to transmitlight, and wherein said prism member has at least one rotationallyasymmetric curved surface.
 40. A finder optical system according toclaim 39, wherein said prism member is arranged such that one of saidsurfaces having a reflecting surface and one of said surfaces having atransmitting action are formed from an identical surface.
 41. A cameraapparatus according to which has said finder optical system, claim 39 or40.
 42. A camera apparatus according to claim 41, which has aphotographic optical system provided separately from said finder opticalsystem.